The present invention relates to a heater unit equipped to a tray stored in a food cart used on board an aircraft.
Heretofore, the food cart used on board an aircraft is cooled as a whole by a chiller and the like, and then a dish on the tray mounting meat, fish and the like that needs to be served warm is heated partially.
There are two types of heater units for heating the dishes. One type mounts individual thermostats, and thereby controls the temperature, and the other type controls the current or voltage supplied thereto using a control box.
On the other hand, the requirements related to the electromagnetic interference caused by the electronic devices on board the aircraft are becoming more serious. If the heater unit utilizes a thermostat for heat control, the unit is required to include a circuit capable of absorbing the electromagnetic wave generated when the device is turned on and off by a ferrite core and the like, and the structure of the heater unit becomes complex.
Further, since the thermostat is relatively large, the thickness of the heater unit is increased and the height of the portion of the tray where the heater unit is positioned becomes too high.
However, the unit utilizing current or voltage control requires a complex control circuit leading to increase of the manufacturing cost.
In order to solve the above-mentioned problems, a heater having a PTC function is proposed, thereby enabling to cut out the thermostat and the like from the heater. However, according to the proposed heater, a considerably long time is required for the heater to rise up to a certain temperature.
The present invention aims at providing a thin-type heater unit having advantageous heat responsibility, that does not require a costly control system and does not generate electromagnetic waves caused by the power being turned on and off.
The present invention utilizes a ceramic heater (hereinafter called a PTC heater) having positive temperature coefficient, characterized in that when if the heater reaches a certain temperature, the electrical resistance increases rapidly and the temperature is maintained at a set value (positive temperature coefficient: PTC). The ceramic heater (PTC heater) is connected in series with a sheet heater, and the heaters are bonded so that heat interference is caused between the ceramic heater and the sheet heater.
According to the present invention, the tray for a service cart used on board an aircraft comprises as means for heating food a plural number of ceramic heaters having a positive temperature coefficient that are connected in parallel within an insulated substrate having advantageous heat resistance, and a sheet heater, wherein the ceramic heaters and the sheet heater are connected in series.
The ceramic heaters and the sheet heater are bonded integrally via an insulating layer, and the surface layer is also covered with a similar insulating layer.
Moreover, the heater unit is adhered integrally on one side thereof with a metal having high thermal conductivity.